Fire tube boilers are used across a broad range of applications, most especially as package boilers that are offered as build-to-stock or build-to-order items that can be shipped complete to or ready for configuration at a user site. Package boilers are frequently used in industrial, commercial, and multi-unit residential applications to provide hot water or steam for a variety of uses.
FIG. 1 is a simplified diagram of a fire tube boiler 100 made according to the prior art. The fire tube boiler 100 includes a shell 102 having a front wall 103, a back wall 105, and a peripheral wall 107 configured to hold water 104. A combustion pipe 106 disposed at least partially inside the shell 102 defines a combustion volume 108 and holds the water 104 out of the combustion volume 108. The combustion pipe 106 can also be referred to as a morrison tube or furnace. A fuel nozzle 110 is disposed to receive fuel from a fuel source 112 and output a fuel jet into the combustion volume 108 and an air source 114 is disposed to output combustion air into the combustion volume 108. The air source 114 can consist essentially of a natural draft air source, or alternatively can receive air from a blower 116. Various fuels are used in commercially available fire tube boilers. For example, the boilers can use natural gas, propane, #2 fuel oil, and/or #6 fuel oil, alone or in combination.
The fuel jet and combustion air together support a conventional flame 118 in the combustion volume 108. The flame 118 produces hot flue gas that is circulated through fire tubes 120, 122 that, together with the wall of the combustion pipe 106, transfer heat produced by the combustion reaction 118 to the water 104. In the illustrative example 100, the fire tubes 120, 122 and the combustion pipe 106, form a three pass system with hot flue gas being produced in the combustion pipe 106 flowing from left to right, a second pass of fire tube 120 supporting flue gas flow from right to left, and a third pass of fire tubes 122 supporting flue gas flow from left to right. Each “turn” of flue gas direction is made in a plenum 124, 126. Various numbers of passes, for example between one (combustion pipe 106 only) and four, are typically used according to the design preferences for a given installation or standard product. The embodiment of FIG. 3 is referred to as a “dry back” boiler. In a “wet back” boiler, the plenum 124 has a wall separate from the back wall 105 with space for boiler water 104 to circulate therebetween.
Cooled flue gas is vented to the atmosphere through an exhaust flue 128. Optionally, the vented flue gas can pass through an economizer that pre-heats the combustion air, the fuel, and/or feed water 130 to the boiler 100. The water 104 can consist essentially of (hot) liquid water (e.g., except for boiling that may occur immediately adjacent to the heat transfer surfaces of the fire tubes 120, 122 and the combustion pipe 106), or can include liquid water and saturated steam 132. The output hot water or steam 132 is transported for use as a heat source for a variety of industrial, commercial, or residential purposes.
An automatic controller 134 may be used to control output of hot water or steam 132 according to demand received via a data interface 136. The controller 134 can control fuel flow using a fuel valve 138 and can control an air damper or blower 116 to match flame 118 heat output, and thereby control heat output to hot water or steam 132 demand. The controller 134 can further control a steam or hot water valve 140 and/or a feed water valve 142 to control the flow rate of water 104 through the boiler 100.
Although most fire tube boilers such as package boilers are relatively low thermal output compared to the range of industrial burners, and therefore can individually be relatively clean sources of hot water or steam 132, they collectively represent a significant source of air pollution owing to a relatively high number of installations.
What is needed is a burner technology that can be applied to a fire tube boiler that will produce a reduced output of pollutants including carbon monoxide (CO) and/or oxides of nitrogen (NOx).